Carbon molecular sieve (CMS) membranes are able to efficiently separate gases that have a small difference in molecular sizes. To withstand the large transmembrane pressures required to attain high gas flux, it is preferred to prepare CMS membranes as composite configurations using inorganic supports. This paper reports on the synthesis and properties of novel CMS membranes on mesoporous gamma-alumina supports. Such supports allow coating of thin, high-quality polymer films and subsequent CMS membranes. Thin CMS membranes were obtained by pyrolysis of defect-free polymer/ceramic composite membranes in an oxygen-free inert gas environment. Compared to macroporous alpha-alumina supported membranes, both polymer and CMS membranes prepared on gamma-alumina show better gas permeation and separation characteristics. A 1.6 mu m thick CMS membrane on gamma-alumina support offers a pure-gas ideal separation factor of 45 for He/N-2 and 5.1 for O-2/N-2. When tested with a 50/50 C3H6/C3H8 mixture, the CMS membrane shows a mixed-gas C3H6/C3H8 selectivity up to 36 with a C3H6 permeance of around 3 x 10(-9) mol/Pa.s.m(2) (9 gpu).